It is common medical practice to administer nitrous oxide to patients in order to anaesthetize them for surgery. Once the patient has been initially anaesthetized, they may be maintained in such a condition by the balanced administration of oxygen and nitrous oxide. If, for some reason, the oxygen supply should fail the anaesthetized patient could receive an excess dose of nitrous oxide which could, if allowed to pass undetected, prove fatal.
People within the medical profession have long realised the possibility of such an occurrence and various members of the medical profession, particularly those amongst the anaesthetists, have studied this problem and have made recommendations concerning the prevention of such an occurrence.
By way of example, the standing sub-committee on safety in anaesthesia of the Association of Anaesthetists of Great Britain and Ireland have published a guide in regard to this matter, which guide recommends inter alia:
The device should be relatively cheap. It should be easily fitted and should be capable of working satisfactorily on all the continuous or intermittent flow machines in common use, whether the oxygen supply is obtained from cylinders or from a pipe-line installation.
The device should preferably be fitted permanently to the machine. If all or part of the device has to be detachable (e.g. for sterilisation) it should be so designed that it is impossible to give an anaesthetic with that machine unless the detachable part has been correctly replaced.
The device should be mounted in an obvious position. If this is not possible a label should be permanently attached to the machine in a prominent place to indicate that the device has been fitted. The label should indicate the purpose, mode of function and method of testing the device and should be easily visible from the front of the machine.
The device should be automatically tested whenever the oxygen supply is switched on or off. Provision for testing at other times may be incorporated if desired.
The energy required to operate the device should be derived solely from the normal oxygen supply pressure in the line between the cylinder or pipe-line inlet to the machine and the oxygen flowmeter control valve.
The device should be activated when this pressure falls to a value which is ideally not less than two-thirds and certainly not less than half of the normal oxygen supply pressure to the flowmeter.
When the device is activated it should give an audible and, if desired, visible warning of the reduction in oxygen supply pressure even if the patient is not breathing.
The audible warning should have a distinctive sound, be clearly audible to a person with normal hearing above a background noise level that may be expected in an operating theatre. The Committee suggests that an 80 dB whistle would be adequate but the necessary experimental work has yet to be done. The sound should last for at least 10 seconds. These requirements should be met whether the oxygen supply pressure failure is caused by cylinder emptying or pipe-line disconnection.
It should not be possible to supply any other gas to the patient unless the oxygen supplies to the flowmeter and oxygen bypass valve are connected and are at their designed operating pressure.
It is therefore an object of the present invention to provide a safety and protection apparatus which will go at least some way in achieving the requirements hereinbefore set forth.